This invention relates to a vibrator for generating mechanical vibration in devices such as a screening machine, a parts feeder for a working or assembling machine, or a product feeder for a weighing or packing machine. The inventive device is especially useful in a dispersion feeder for distributively feeding product to be weighed to respective weigh hoppers of a combination weigher.
One type of such vibrator is disclosed in Japanese utility model opening gazette No. 59-52426. In this device, the vibration is generated by a motor-driven cam wheel which is coupled to a member to be vibrated. The amplitude of vibration in this device cannot be controlled easily since it is determined by the profile of the cam wheel.
Another type is disclosed in Japanese patent publication No. 58-18283. In this device, a circular table to be vibrated is supported at its periphery by a plurality of leaf springs and a helical vibration is applied thereto by electromagnets energized by a.c. power. This device has a disadvantage in that the vibration amplitude is substantially zero at the center of the table and increases radially outwardly, though it can be easily controlled.
A further type of vibrator disclosed in Japanese utility model publication No. 49-36289 includes an eccentric weight rotationally driven by a motor which is fixed to a member to be vibrated. In this device, the member, together with the motor, is subjected to a revolving vibration (i.e., orbital movement) caused by centrifugal force generated by rotation of the eccentric weight. This device also has a problem in that it is difficult if not impossible to control the amplitude of vibration during operation. Since simple and accurate adjustment, even though theoretically possible, is difficult in actual practice for an average operator, the amplitude normally remains fixed.